Or How To Keep Your Wood Prop On Your Plane With Less Work · • Burt Rutan proposal 25 years ago...

Rutan Derivative Canard Forum Slide #1July 19, 2007

A Method of Desensitizing Wooden Propeller Hub Mounting Bolt Security To Environmental Effects and

Sub-Optimal Maintenance TechniquesOr

How To Keep Your Wood Prop On Your Plane With Less Work

Marc J. ZeitlinJuly 26th, 2007

2:30 PM – 3:45 PMForum Tent 06 – Sporty’s Pavilion

July 19, 2007 Rutan Derivative Canard Forum Slide #2

What Will I Talk About?• Characteristics of Wood Props• Need for Maintenance• Proposed Solution – Representation / Theory• Specific Solution• Prior Art / Proposals• Pre-Flight Testing• Flight Testing• Current Configuration• Increased Robustness / Decreased Sensitivity• Cost / Weight Penalty• Questions / Answers / Availability


Characteristics of Wood Props

• Relatively soft• Low crush strength• Low stiffness• Susceptible to

moisture / humidity / shrinkage / expansion


Need For MaintenanceExamples of Wooden Propellers:• Sensenich – 50 hrs or annually• Hertzler – every 25 hours, anytime going

between different climates• Jabiru – every 25 hours• Miles Gemini – every 10 hours

Technique (not usually specified):• Remove safety wire• Loosen bolt first to break static torque• Ensure movement – measure torque while turning


Proposed Solution –Representation / Theory - 1• Prop Bolts very stiff – Prop Hub size determines force

Prop Hub(Somewhat Squishy)

Very Stiff Spring

(Prop Bolt)

Hub shrinks, compression force falls, bolts cannot

compensate

Before Compression After Compression After Shrinkage


Proposed Solution –Representation / Theory - 2• Same motion of hub• Small change in spring force – SOFTER SPRING DETERMINES FORCE CHANGE

Soft Spring

Before Compression After Compression After Shrinkage


Specific Solution

• Belleville Washers• Flexible arrangements

– use serial stack• Force/deflection

curves adjustable –can obtain any force/deflection required


Prior Art / Proposals• Jabiru - recently switched to belleville stacks on certain

engine prop combinations

• Miles M65 Gemini w/Cirrus Minor II Engines, from 1947 – used belleville stacks

• Paul Lipps / Vance Jaqua – belleville stacks installed on Lancair 235 w/O-320

• Burt Rutan proposal 25 years ago – never tested

• Independently proposed to me by 3 sources – c-a mailing list; web forum; co-worker


COZY MKIV Installation

• Took proposals, did calculations, picked hardware

• Determined 6-stack would provide more than adequate expansion / shrinkage capability


Pre-Flight Testing - 1• Find

Hardware

• Two bolts have stacks of 6 bellevilles captured between two wide area washers – 4 standard bolts

• Can see good linearity

Bolt Torque / Washer Compression vs. Turns

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5 4

Bolt Turns

Bol

t Tor

que

(ft-lb

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Was

her C

ompr

essi

on -

(in)

Torque - LTorque - RCompression - LCompression - R


Pre-Flight Testing - 2

• Remove “turns” from the equation – just show washer compression vs. applied torque

• Still very linear

Washer Compression vs. Bolt Torque

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 5 10 15 20 25 30 35

Bolt Torque (ft-lb)

Tota

l Was

her C

ompr

essi

on (i

n)

Compression - LCompression - R


Pre-Flight Testing - 3• Switch from six

std. bolts to six bolts – all with Belleville washer 6-stacks

• Change in slope as “slop” is taken out of system (both types of bolting systems) – then more torque for same compression needed

Hub Compression vs. Bolt Torque

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 5 10 15 20 25 30

Bolt Torque (ft-lb)

Hub

+ F

lang

e th

ickn

ess

(in)

Compression - StdCompression - Belleville


Pre-Flight Testing - 4

• Six bolts with belleville washer 6-stacks

• Very linear torque and compression with bolt turns

Washer / Hub Compression / Bolt Torque vs. Bolt Turns

0

5

10

15

20

25

30

35

0 1 2 3 4 5

Bolt Turns

Torq

ue (f

t-lb)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Com

pres

sion

(in)

Torque

Hub Compression

Individual WasherCompression


Pre-Flight Testing - 5• Same slight

change in slope as “slop”is taken out

• Still predictable and relatively linear with Bolt Torque

• Might be easier to measure turns rather than torque, however

Hub and Washer Compression vs. Bolt Torque

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0 5 10 15 20 25 30

Bolt Torque (ft-lb)

Com

pres

sion

(in)

Hub CompressionIndividual Washer Compression


Flight Testing - 1• First few flights – two bolts with bellevilles – four standard bolts• Calcs show prop will stay even if two bolts don’t• See washers NOT completely compressed – allows for shrinkage AND expansion of

Prop Hub

• No difference from standard installation perceived - ~3 flight hours accumulated in this configuration at various speeds, RPM, flight attitudes – near 1 million cycles for fatigue


Flight Testing - 2• Next few flights – four sets of belleville washers, two standard bolts• Have confidence based on previous flight hours• Run another ~3 hours – build ~2 million cycles on first set, ~1 million on

second set• Again, fly various RPMs, flight attitudes, speeds, G-loading


Flight Testing - 3• Next set of flights – all six bolts with belleville washer stacks• Built more confidence based on previous flights• Run another ~3 hours – build ~3 million cycles on first set, ~2 million on

second set, ~1 million on third set• Again, fly various RPMs, flight attitudes, speeds, G-loading


Flight Testing - 4• Remove two bellevilles from each stack – now have 4/bolt• Run another ~40 hours – build ~13 -15 million cycles on all belleville

washers• Again, fly various RPMs, flight attitudes, speeds, G-loading


Current Configuration

• Using 7” bolts – could reduce to 6.5” and remove hardened washer to save ~0.1 lb.

• Added AN3 bolts to FRONT of crush plate

• Will be able to add spinner backplate / mounting structure WITHOUT having to remove/touch prop bolts

• Don’t have to have backplate BETWEEN prop and prop extension


Increased Robustness / Decreased Sensitivity

• Maintenance simplification (measure spacing, # turns)

• Maintenance frequency reduction (Jabiru – from 25 hrs. to 1/yr. example)

• Safety enhanced

Hub Compression Force Loss vs. Hub Shrinkage

0%

10%

20%

30%

40%

50%

60%

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Hub Shrinkage (in)

Hub

Com

pres

sion

For

ce L

oss

(%)

Standard

Bellevilles (6-stack)




Cost / Weight Penalty

• ~3.5 lb for ½” Bolt SAE-2 Flange with 7” Crush Plate and 6 washers/stack (~2.5 lb in current configuration – 4 washers/stack)

• Approximately $120 gets replacement bolts, bellevilles, flat washers


Questions / Answers / Availability• My Email: [email protected]

• Website: http://www.cozybuilders.org/

• Currently ONLY configured for ½” bolts – 3/8”bolt configurations may be tested in the future.

• I can supply Bellevilles at cost+; Bolts available from Saber MFG, washers from ACS, Wicks, etc.

Or How To Keep Your Wood Prop On Your Plane With Less Work · • Burt Rutan proposal 25 years ago...

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Transcript of Or How To Keep Your Wood Prop On Your Plane With Less Work · • Burt Rutan proposal 25 years ago...